Power transmission using a high-voltage direct current link (HVdc) has been considered for decades to interconnect two points located at a significant distance where this approach becomes more cost effective than alternating current (ac) transmission. Currently, dc transmission is being considered for applications such as connecting two separated ac systems and renewable resources like wind farms to the load centers, particularly, off-shore wind farms. With the development of efficient and faster power electronic components and the reduction in the production cost, the use of dc transmission will increase in the future.
Conventionally, there are two HVdc topologies: thyristor-based line commutated converters (LCCs), and voltage source conveners (VSC), which currently use silicon (Si) isolated gate bipolar transistors (IGBTs). Within the VSC, the modular multilevel converter (MMC), which in general has a half-bridge in each module, has recently become the preferable solution for VSC-HVdc due to its multiple advantages when compared to the other VSC topologies. Regardless of the type of HVdc topology, a three-phase fundamental-frequency transformer is usually used at the ac side to match the ac system.
In the last few decades, high-frequency transformers (HF-XFMR) and medium-frequency transformers (MF-XFMR) have been proposed to replace the fundamental-frequency transformer for utility scale applications. Advantages such as volume and weight reduction make these types of transformers attractive for particular applications when compared with the fundamental-frequency transformer. When these transformers are used, an ac-to-ac converter is required to reduce the HF or MF to utility levels (i.e., 50/60 Hz).
Matrix converters (MC) have been extensively analyzed as an alternative to convert ac-to-ac power without using large energy storage components in the dc link as in the conventional back-to-back converter (BBC). Advantages for these devices are unity of input/output power factor, bidirectional power flow, sinusoidal input current and output voltage, and the lack of a bulky and lifetime-limited electrolytic capacitor in the dc link, which results not only in a substantial converter size reduction, but potentially, in a more reliable system.
There are two types or MCs: conventional matrix converters (CMCs) and indirect matrix converters (IMCs). The CMC uses nine bidirectional switching positions, normally realized as two unidirectional switching devices in either a common-source or common-emitter configuration, to allow a direct connection between the input and the output. The IMC, also called the dual-bridge topology, separates the ac-ac conversion into rectifier and inverter power stages, and thus, allows independent control of the two stages, overcoming the complex MC control requirements. The IMC presented in FIG. 1 has two main stages: an input filter 100 connected to rectifier stage 102 which uses 12 unidirectional devices 103 for applications that require bidirectional capability, such as motor drives or certain distributed generators (e.g., microturbines), and the inverter stage 104 connected to output filter 106 which is a conventional two-level three-phase voltage source converter (VSC) requiring 6 unidirectional switches 108.
Distributed generation systems inject power into the utility system, so the electrolytic capacitors do not play the same role as in the case of a motor drive. Hence, the IMC advantages make it a viable alternative to be considered as a standard power electronic interface (PEI) for distributed generation systems. In general, conventional Si IGBTs and diodes are used to build MCs. Recently, reverse blocking IGBTs (RB-IGBTs), which are considered to be a bidirectional switch, are being used for further size reduction. New wide bandgap power semiconductor devices, such as SiC JFETs and SiC MOSFETs are becoming more competitive for industrial applications that require high power densities and operate in extreme environments. Advantages such as low on-resistance, high temperature capability, reduced cooling requirements, and high breakdown voltages make them the preferable switching devices. The use of SiC devices allows for the increase in converter switching frequency, fsw, which helps to reduce size and weight of the overall converter.